This invention relates to tensioners used with chain drives in automotive timing applications and more particularly to a coil spring chain tensioner.
Tensioning devices are used as a control device for a power transmission chain, or any similar power transmission devices, as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of such slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage will throw off the camshaft timing by several degrees, possibly rendering the engine inoperative or causing damage. However, in the harsh environment in which an internal combustion engine operates, chain tension can vary between excessively high or low levels as a result of the wide variations in temperatures and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Moreover, wear to the chain components, during prolonged use, can result in a decrease in the tension of the chain. In addition, cam shaft and crank shaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation, determined by chain stiffness.
There are several types of chain tensioners which can be used to tension a chain. Two common types of chain tensioners are blade-type chain tensioners and coil spring chain tensioners.
Generally, blade-type chain tensioners utilize a blade spring interlocked under tension with a shoe to provide tension to a chain. Specifically, the blade spring is arcuate in shape and the shoe is relatively flat. The shoe is further constructed from a material that will deform or "creep" upon experiencing a load at a high temperature. The blade spring is bent to correspond to the shape of the shoe and interlocked therewith. Through the bias of the blade spring, which wishes to return to its original, more arcuate shape, a load is applied to the shoe. Thus, as the operation of the engine causes the temperature of the shoe to increase, the load from the blade spring causes the shoe to deform to a more arcuate shape. Through such deformation, tension is provided to a chain. Specifically, the blade assembly is positioned relative to the chain so that as the shoe becomes more arcuate it bears upon a span of chain and increases chain tension.
There are several disadvantages with using a blade-type chain tensioner to tension a chain. For instance, blade-type chain tensioners are generally not very compact in size. Thus, blade-type chain tensioners may not be used in some applications with tight space requirements. In addition, blade springs can only bias a shoe a short distance. Therefore, blade-type chain tensioners cannot exert a very high tension force on the chain.
Coil spring chain tensioners overcome some of the disadvantages of blade-type chain tensioners. Instead of using a blade spring to bias the shoe, coil spring chain tensioners use the ends of a coil spring to bias the shoe. Coil springs are capable of biasing a shoe a greater distance than that of a blade spring. Therefore, coil spring chain tensioners can exert a greater tension force on a chain than the blade-type chain tensioners. In addition, coil spring chain tensioners can be compact in size.
An example of a coil spring chain tensioner is the Buick 3800 Coil Spring Tensioner ("3800 Tensioner"), which is manufactured by the assignee of this invention. The 3800 Tensioner has a tensioning shoe pivotally mounted between one end of outer and inner brackets with a pivot pin. The other end of the outer and inner brackets is pivotally mounted to a fixed bushing.
In order to impart a force on the shoe, a coil spring having first and second elongated ends is wound about the bushing. The first elongated end extends from the coil spring in a direction perpendicular to the bushing and contacts the shoe. The second elongated end extends from the coil spring in a direction parallel to the bushing and is fixed relative to the bushing. Since the second elongated end is fixed, the first elongated end imparts a rotational spring force on the shoe as the coil spring tries to unwind.
In the present invention, however, both ends of the coil spring are used to bias the shoe and tension the chain. Thus, the coil spring imparts a translational spring force on the shoe and biases the shoe in a direction perpendicular to the windings of the coil spring. As a result, the coil spring is capable of biasing the shoe a greater distance than the 3800 Tensioner for a given amount of spring force. Moreover, the present invention has only three basic elements and does not include pivot pins or inner and outer brackets to attach the shoe to the bushing.
Accordingly, it is an object of the present invention to provide a coil spring chain tensioner which is an improvement over the prior art coil spring chain tensioners and overcomes the previously mentioned disadvantages associated with a blade-type chain tensioner.